Process for the production of high-alpha-cellulose fiber



July 24, 1928.

G. A. RICHTER PROCESS FOR THE PRODUCTION OF HIGH ALPHA CELLULOSE FIBER Original Filed April 28, 1925 to the bleachery, The Wash water used in the washer assumes practically the same temperature as the hot spent digesting liquor and the hot pulp, as they are passed into the washer. The hot wash water (which for convenience of designation I term the alpha spent liquor) contains substantially all the sodium constituent in any form,vspent or otherwise, associated with the alpha stock as it was passed into the washer. This hot wash water is re-circulated to the digesting tank A, and comprises in volume approximately the volume of water associated with the pulp after digestion, plus the total volume of wash water used in the washer, minus that portion absorbed by the washed alpha pulp as it passes to the bleachery. During this stage of the process, the volume of wash water employed in washing the digested stock is so regulated that thevolume of spent alpha liquor obtained from the washer and re-circulated into the tank A is substantially equal'to the volume of water and digesting solution employed in making up the initial 10% stock suspension in the tank A. To this recirculated alpha spent liquor is continuously added a suicient quantity of 50% sulphite pulp, and the amount of concentrated caustic soda or soda ash solution necessary to effectively remove the impurities contained in the sulphite pulp,-thereby again producing a stock suspension of about 10%. The 10% stock suspension formed f with the recirculated spent liquor is digested, and the cycle of operationsas hereinbefore described in connection with the initial 10% stock suspension, is repeated. The hot spent alpha liquor obtained from the washer at this point in the process contains a higher per cent of dissolved material. This liquor is rei-circulated as before, and the process, as described, is continued or repeated until a hot, syrupy spent liquor of about 10 to 15fBaum results. Such a concentrated spent liquor may profitably be evaporated, and the combustible content thereof burned, to recover the valuable sodium constituent.

When this concentration of spent liquor is reached, the flow of fresh water to the washer vis increased. As stated previously, up to this point the volume of spent liquor delivered from the washer has been only suficient to dilute the incoming unbleached sulphite stock to a 10% consistency, or suiicient to ensure such consistency when the volume of solution containing fresh di esting constitutent is taken into account; ut there is now an excess of spent liquor. This excess is continuouslywithdrawn from the spent alpha liquor obtained from the washer and 1 mixed with the black liquor` resulting from a soda pulp digestion as 1t 1s eing passed to the evaporators. The rest of t e spent alpha liquor is re-circulated to the alpha di estin tank A. The increase in quantity o was soda digestion.

water,-that is, the excess passed along with the black liquor to the evaporators,-is so adjusted as to maintain an equilibrium within the system at this particular concentration: that is, the excess spent liquor mixed with the spent soda liquor contains substantially the amount of spent sodium constituent which was formed in the preceding digestion (the term sodium or sodium constituent as used in this specification and the appended claims, will, of course, be understood to mean sodium in combined forni,- that is, lin the form of a sodium compound).

It will of course be further understood that the spent black liquor referred'to, as indicated on the drawing, results from the digestion of wood chips under the requisite conditions of temperature and pressure, in a digester, in the usual soda digesting liquor, which consists^ essentially of a solution of caustic soda and a relatively small amount of soda ash and traces of impurities. After the necessary period of digestion, the soda digester contents are blown into the usual blow pit, and the soda pulp is then passed through a counter-current washer similar to the one in which the high alpha-cellulose pulp is washed. FThe Wash water from the counter-current washer contains substantially `all the digesting constituent, spent or otherwise, associated with the soda pulp as it is passed into the washer, is black, and of about a 14 Baum consistency. In certain cases, it may be desirable to return a relatively small portion of this spent black liquor to the digesters for use in a subsequent The large portion of black spent soda liquor, however, is passed into a storage tank, from which it is' continually withdrawn to the evaporators. To this blackliquor, as has been already indicated, is added the excessspent liquor of from 10 to 15 Baume, obtained from the alpha spent liquor, so that a mixture of alpha and black liquors of aboutl the samedegree Baume is passed into the evaporators for concentration. The combustible content of the resulting concentrate is burned, preferably in a combined boiler and smelting furnace, in an oxidizing atmosphere, in the usual manner. The smelt from the furnace consists essentially entirel of sodium carbonate, and is run into a issolving tank containing just enough water to formv a concentrated boiling solution of. sodium carbonate. 'The amount of sodium carbonate solution which together with the unspent sodium constituent present in the re-employed alpha spent liquor is necessary for effective digestion lof the unbleached sulphite stock fed into the di esting tank A, is continuously withdrawn rom the'smelt solution, is filtered in the usual `rllhe remainder of the smelt solution is passed into a causticizing tank Wherein the sodiumA carbonate is converted into caustic soda with limer' The precipitated calcium carbonate may then be removed by filtration, and the clear vWhite caustic soda solution obtained may be employed in effecting the next soda pulp digestion. Losses in both the alpha and the soda digestion cycles may be made up by the addition oi suicient soda ash to the smelt solution prior to causticiaation. With caustic soda more readily available, as in a mill `Where caustic soda is produced by the electrolysis oit salt (NaCl) solution, the losses may be made up b the addition ot caustic soda to the White liquor subsequent to causticization as ,it is passed to the soda digester.

Since sodium carbonate cooking liquor is as edective as caustic soda dige/sting liquor in the production ol high alpha-cellulose liber, the hereinbelore described process, vvhi'le possessing all the advantages incident to the co-pending application and eliminating the necessity for special recovery apparatus in an alpha-cellulose fiber plant, als etfects an economy in lime equivalent to that till which otherwise would be necessary to causticize an amount of soda 'ash equivalent to the losses experienced in the alpha digesting cycle: that is to say, it the losses in the alpha .digestion cycle Were added in the torno ot caustic soda on the alpha side ol the dravving, instead of on the sodalside ot the drawing, morey lime `would be consumed in the causticization than is consumed b the method described. @il course, the met od oi' recover of the sodium constituent trom the alpha fiber plant When practiced in connection with the recover ot sodium constituent from the soda plant is susceptible ot various modifications which are at -once apparent to those skilled in the art, and which lie Within the spirit ot this invention as defined by the appended claims and, iWhile ll have conined myseliE to the description oi' what is the preilerred method oit practicing such recovery in. a particular instancenit is not the intention that this description shall have a limiting edect on the invention.

ll do not herein claim features generic to` application, Serial No.. Il32,522, nled December l, 1925, `by Milton Schur and myself, nor do l herein claim lthe process disclosed in m applications, Serial No. 26,3%, tiled Apri 28, i925, and Serial No. 28,339, tiled May 6, 192e. y

What ll claim isi l. A. process vvhich comprises digesting predigested pulp in an alkaline liquor containing sodium compounds, ria-employing one portion of the spent liquor in another digestion ot like pulp and recovering the so ium compounds in the other portion ot spent liquor, together with the 'sodium compounds contained in the spent liquor resulting from the production of pulp from raw cellulosic material.

2. A process which comprises digesting predigested pulp in an alkaline liquor containing sodium compounds, re-employing one portion of the spent liquor in another digestion of like pulp, and mixing the other portion of spent liquor containing substantially' the amount ot spent sodium formed in a previous digestion with spent liquor containing sodium compounds resulting from a digestionotl raw cellulosic material, recovering the sodium constituent of said mixed liquors, using one part of the recovered sodium constituent necessary to edect another digestion of predigested pulp with thereemployed portion of spent liquor, and using the other partqin the production ot pulp trom raw cellulosic material after the addition thereto ot' sucient sodium constitulitent to make up losses suffered in both digesting cycles.

3. A process lwhich comprises digesting unbleached sulphite pulp in an alkaline liquor, re-employing one portion of the hot spent liquor in another digestion of like un- !oleached sulphite pulp, passing thev other portion, together with the spent liquor from a .soda pulp digestion, into the recovery system oi the soda plant, and recovering the sodium constituent'of the mixture. l

4. A proces-s which comprises digesting predigested pulp in an alkaline digesting liquor containing sodium compounds, reemploying one portion of the hot spent liquor in Yanother digestion ot like pulp, mixing the other portion containing an amount of spent sodiujm equivalent to that which was formed in a preceding digestion with the spent liquor containing sodium compounds resulting from the digestion ot raw cellulosic material, receiving the sodium constituent ol the minture, and adding to said re-employed portion the recovered sodium constituent necessary for another digestion of predigested ul` p epfl process which comprises digesting predigested pulp in an alkaline digesting liquor containing sodium compounds, reemploying substantially allfthe spent liquor of one digestion in another digestion alter the addition ot sucient alkaline agent thereto to make up losses therein tor etlective digestion, continuing such reemployment until a predetermined sodium concentration results therein, and thereafter reemploying one portion ol the spent liquor in another digestion of like pulp, mixing the other portion containing an amount of sodium equivalent to that which was formed in a previous digestion with liquor containing sodium compounds, resulting from the digestion of raw cellulosic material, recovering the sodium constituent ont the mixture, an l lll@ .lill

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adding to said re-employed portion of spent duction of soda pulp together with the spent liquor y resulting from digestion of predigested pulp with alkaline liquor.

l8. A process which comprises digesting sulphite pulp under heat in an alkaline liquor containing sodium compounds, mixing a portion of the spent liquor resulting from such digestion with the spent black liquor resulting from soda 'digestion of raw cellu-l losic material, recovering the sodium constituent from the mixture, and using part of sucli recovered sodium constituent in another digestion of sulp'hte pulp and part for the digestion of raw cellulosic material.

9. A process which comprises digesting predigested pulp under heat in an alkaline liquor containing sodium compounds, re-employing one portion of the hot spent liquor of such digestion in another digestion of predigested pulp, mixing the other portion with the spent black liquor resulting from soda digestion of raw cellulosic material, recoveringthe` sodium constituent 'of the mixture, and using part of said recovered sodium constituent in the digestion of raw cellulosicmaterial and part iii-another digestion of predigested pulp together with said re-employed portion of spent liquor.

10. A process which comprises digesting predigested pulp under heat inan alkaline liquor containing sodium compounds, re-employing one portion of the hot spent liquor resulting'from digestion in an'other digestion of predigested pulp, mixing the other portion containing substantially the amount of spent sodium constituent formed in a preceding digestion with -the spent black liquorresulting from soda digestion of raw cellulosic material, recovering the sodium constituent of the mixture in the form ofsodium action, circulating a part of the hot Washv Water back for further digestion of pulp, mixing the other part With the b lack spent liquor resulting from a soda digestion, evaporating said mixed spent liquor, burning the organic content and smelting the inorganic content thereof in an oxidizing atmosphere so as to Agive a smelt consisting essentially of sodium carbonate, recovering the molten smelt in Water, mixing one portion of the resulting hot liquor with the recirculated Wash Water, causticizing the remaining portion With lime, filtering it, re-employing said filtered white liquor in a subsequent soda pulp digestion, and making up losses 1n both digesting cycles by the addition ofsodium digesting agent to said filtered liquor.

In testimony whereof I have aiixed my signature.

QEoRGE A. RICHTER.

l pulp and the entrained products` of the re- 

